1. Field of the Invention
The present invention relates to a wiring board comprising an insulating substrate containing at least an organic resin, and via-hole conductors formed by filling the via-holes with an electrically conducting material, in order to decrease the resistance of the via-hole conductors and to improve the reliability of connection.
2. Description of the Prior Art
In recent years, a so-called printed board having an electrically conducting wiring layer formed on the surface of an insulating substrate that contains a thermosetting resin such as epoxy resin or phenol resin, has been applied to a circuit board and a package that mounts a semiconductor device. In order to form an electrically conducting wiring layer on the printed board, there have been employed a method by which a copper foil is adhered onto the surface of an insulating substrate which is then etched to form a wiring pattern, a method by which a copper foil forming a wiring pattern is transferred onto the insulating substrate, and a method by which a wiring pattern is formed on the surface of the insulating substrate based on a metal-plating method.
As the wirings are formed in many layers, further, it has been attempted to electrically connect different electrically conducting wiring layers using via-hole conductors. Generally, however, the via-hole conductors are formed by forming via-holes in the insulating substrates of the multi-layer wiring board at predetermined portions by using a drill or the like and plating a metal onto the inner walls of the via-holes.
According to the above-mentioned methods, however, expensive chemicals are used for conducting the chemical plating, and extended periods of time are required for the treatment, offering difficulty in the production and economy. Further, the via-hole conductors obtained by plating a metal on the inner walls cannot be easily formed among the layers in the multi-layer structure arousing a problem in that it is not possible to increase the density of the conductor wiring layers.
To cope with such a problem in recent years, there have been proposed methods in which electrically conducting wiring layers are formed in many layers by coating the surface of the insulating substrate with, and by filling the via-holes with, an electrically conducting paste obtained by mixing a metal powder such as of silver, copper or solder and a thermosetting resin or an activating agent as taught in Japanese Patent No. 2603053, Japanese Examined Patent Publication (Kokoku) No. 39360/1993 and Japanese Unexamined Patent Publication (Kokai) No. 160072/1980.
According to the conventional method of forming via-hole conductors by filling an electrically conducting paste, however, a thermosetting resin component is contained in large amounts in the electrically conducting paste and the contact is not sufficient among metal particles in the via-hole conductors; i.e., the electric conductivity of the via-hole conductors is low.
The object of the present invention, therefore, is to provide a wiring board having an insulating substrate equipped with via-hole conductors that have a high electric conductivity and can be easily formed in any place, featuring very high reliability in the electric connection among the electrically conducting wiring layers.
Another object of the present invention is to provide a wiring board having an insulating substrate equipped with via-hole conductors that exhibit the above-mentioned properties as well as excellent heat resistance.
According to the present invention, there is provided a wiring board comprising an insulating substrate containing at least an organic resin, a plurality of electrically conducting wiring layers formed on the surface and/or inside of said insulting substrate, and via-hole conductors formed in said insulating substrate in order to electrically connect the plurality of electrically conducting wiring layers, wherein said via-hole conductors contain an organic binder as well as a Cu--Sn intermetallic compound as an electrically conducting component.
The Cu--Sn intermetallic compounds include Cu.sub.3 Sn and Cu.sub.6 Sn.sub.5. In the present invention, however, Cu.sub.3 Sn is particularly preferably used. That is, in the wiring board of the present invention, the Cu--Sn intermetallic compound is formed as an electrically conducting component in the via-hole conductors. Therefore, the via-hole conductors exhibit a high electric conductivity, e.g., a volume resistivity of not larger than 1.times.10.sup.-5 .OMEGA.-cm, offering a very high reliability in the electric connection among the electrically conducting wiring layers. Besides, the Cu--Sn intermetallic compound exhibits excellent heat resistance. When the wiring board is maintained, for example, at 260.degree. C. for two minutes, the volume resistivity is maintained to be not larger than 1.times.10.sup.-4 .OMEGA.-cm. Therefore, the connection condition between the via-hole conductors and the electrically conducting wiring layers does not change and a stably connected state is maintained even when the wiring board is heated at high temperatures during the step of reflowing or during the testing of reliability. When a low-melting metal or alloy such as silver or solder is used as an electrically conducting material, the connection condition between the via-hole conductors and the electrically conducting wiring layers changes and the reliability of connection decreases due to a high temperature during the step of reflowing or during the testing of reliability. According to the present invention, however, such a problem is effectively solved.